Cast Iron. 73 Silicon exists in combination with oxygen as silicon dioxide or silica (SiO2), and is so found in the crystals of sea-sand; glass is a mixture of several silicates. It is of value in cast iron. Iron is found in combination with oxygen, the ore being termed a ferric oxide (Fe2 Os), but it may be rendered quite pure by chemical and mechanical means. Sulphur is well known in the form of brimstone, and is an impurity in iron, producing red-shortness. Phosphorus is also an impurity, producing cold-shortness, while Manganese is of value only when mixed with iron and other metals in certain definite proportions. (x.) Cast Iron.—There are seven varieties of iron ore, con- taining from fifty to seventy per cent, of iron in their composition. The blast-furnace (Fig. 84) is used for smelting the ore, which is Jl Xees-SjLdbe/ F/wrnace, done at a very high temperature, with coke as fuel, and lime or clay as a flux.* The molten iron is run into pigs, while the slag * Lime is the usual flux, but clay is sometimes required, as in the case of hematite ore, and then is applied in the form of clay ironstone.